1. Field of the Invention
The present invention relates to a process for removing nitrogen oxides, particularly dinitrogen monoxide (N2O) in exhaust gases.
2. Related Art
The selective catalytic reduction process and adsorption process are known for removing nitrogen oxides in exhaust gases discharged from incinerators, boilers and the like.
The selective catalytic reduction process involves adding a reducing agent in the presence of a catalyst to reduce nitrogen oxides (NOx) into a nitrogen gas (N2). The reducing agent preferably used is ammonia (NH3), which selectively reduces nitrogen monoxide (NO) or nitrogen dioxide (NO2) into a nitrogen gas (N2) and water (H2O). The catalyst used includes noble metal catalysts such as palladium and vanadium catalysts such as V2O5xe2x80x94WO3xe2x80x94TiO2.
On the other hand, the adsorption process involves using a molecular sieve, activated carbon, metal oxide or the like, which physically or chemically adsorbs NO and NO2.
Nitrogen oxides in exhaust gases discharged from incinerators, boilers and the like predominantly contain nitrogen monoxide (NO) and nitrogen dioxide (NO2) with a minor amount of dinitrogen monoxide (N2O). For example, exhaust combustion gases from incinerators may contain 100 ppm of NO, 10 ppm of NO2 and 10 ppm of N2O. Therefore, conventional denitration processes put emphasis on removing nitrogen monoxide and nitrogen dioxide, but overlook removal of dinitrogen monoxide (N2O).
A process for producing semiconductors include a step of performing chemical vapor deposition, which may refer to CVD hereinafter, on wafers. The chemical vapor deposition process sometimes discharges an exhaust gas containing dinitrogen monoxide (N2O) in addition to nitrogen monoxide (NO) and nitrogen dioxide (NO2).
However, the selective catalytic reduction process may produce dinitrogen monoxide (N2O) as a by-product during reduction of nitrogen monoxide (NO) and nitrogen dioxide (NO2) under inappropriately selected conditions. Moreover, the above-mentioned vanadium catalysts are effective to reduce nitrogen monoxide (NO) and nitrogen dioxide (NO2) but ineffective for reducing dinitrogen monoxide (N2O).
The adsorption process can adsorb nitrogen monoxide (NO) and nitrogen dioxide (NO2), but not dinitrogen monoxide (N2O). Moreover, adsorbents used in the adsorption process are readily consumed, and the consumed adsorbents must be replaced. The frequency of replacing adsorbents increases in proportion to an amount of NO and NO2 discharged, resulting in a high running cost.
JPA No. 7826/88 discloses a process for removing dinitrogen monoxide using a palladium catalyst or the like. However, this process removes dinitrogen monoxide after nitrogen monoxide and nitrogen dioxide have preliminarily been removed. This requires a bulky apparatus for removal in at least two steps with lowered heat efficiency. Thus, there is a demand for removing dinitrogen monoxide, nitrogen monoxide and nitrogen dioxide in one step.
Therefore, an object of the present invention is to provide a process for treating an exhaust gas containing dinitrogen monoxide (N2O), nitrogen monoxide (NO) and nitrogen dioxide (NO2) without producing dinitrogen monoxide (N2O) as a by-product.
The present invention provides a process for removing nitrogen oxides in an exhaust gas, comprising the steps of:
mixing an exhaust gas generated during the chemical vapor deposition process for producing semiconductors and containing dinitrogen monoxide (N2O), nitrogen monoxide (NO) and nitrogen dioxide (NO2) with ammonia (NH3) in an amount 0.5-3 times a sum of a stoichiometric amount corresponding to the nitrogen monoxide and a stoichiometric amount corresponding to the nitrogen dioxide; and
contacting said mixed gas with a noble metal catalyst at a sufficiently high temperature to decompose dinitrogen monoxide, nitrogen monoxide and nitrogen dioxide.
Preferably, the present process further comprises, before said contacting step, the step of heating said exhaust gas or said mixed gas at a temperature ranging from 200xc2x0 C. to said high temperature selected for said contacting step.
During said mixing step, said exhaust gas is preferably mixed with ammonia (NH3) in an amount 0.6-2 times the sum of the stoichiometric amount corresponding to nitrogen monoxide and the stoichiometric amount corresponding to nitrogen dioxide.
Preferably, said mixed gas contains 1 mole part or more of ammonia (NH3) per 1 mole part of an oxygen gas (O2).
Preferably, said exhaust gas contains 1 part by weight of dinitrogen monoxide (N2O), 0.01-3 parts by weight of nitrogen monoxide (NO) and 0.01-100 parts by weight of nitrogen dioxide (NO2).
Preferably, said exhaust gas contains 70% by weight or more, more preferably 80% by weight or more of nitrogen gas.
Also preferably, said exhaust gas is substantially free from sulfur oxides.
Preferably, said noble metal catalyst comprises palladium or platinum.
More preferably, said noble metal catalyst comprises a carrier in a particle form and palladium or platinum supported on said carrier.
Preferably, said contacting step takes place at 250xc2x0 C. -600xc2x0 C.
More preferably, said contacting step takes place at 350xc2x0 C.-400xc2x0 C.
During said mixing step, the outlet of said exhaust gas and the outlet of said ammonia preferably face to each other.
Preferably, the present process further comprises the step of passing said mixed gas through a tortuous path after said mixing step and before said contacting step.
Preferably, the present process further comprises the step of removing excessive ammonia after said contacting step.